The present invention relates to an image forming apparatus for forming an image by transferring a color medium from a transfer medium to a sheet in accordance with an image pattern and, more specifically, to an image forming apparatus in which a sheet is reciprocated in a transfer position, and the color media of several colors are transferred in layers onto the sheet, thereby forming a color image thereon.
Among conventional color image forming apparatuses of this type are thermal head printers in which color media are melted by heat in accordance with an image pattern and transferred to a sheet.
Generally, small-sized, low-priced, noise-free, and capable of printing on ordinary paper, these thermal head printers have recently been used as output devices of computers and word processors, and in copying apparatuses.
In one such prior art thermal head printer 1, as shown in FIGS. 1 and 2, a recording head 3 is opposed to a platen 2, and, in the transfer operation, a sheet 4 and a ribbon (transfer medium) 5 are interposed between the platen 2 and the recording head 3. Pressure rollers (pressure means) 6 and 7 are arranged on either side of a transfer position R along the sheet conveying direction, whereby the sheet 4 is pressed against the platen 2 so as to be securely held between the platen 2 and the pressure rollers 6 and 7. A guide roller 8 and a separating roller (separating means) 9 are arranged between the transfer position R and the pressure rollers 6 and 7, respectively. The guide roller 8 serves to guide the ribbon 5 so that the ribbon 5 is joined with the sheet 4 in the transfer position R, while the separating roller 9 serves to guide the ribbon 5 so that the ribbon 5 is separated from the sheet 4 after the transfer operation. In the transfer operation, the sheet 4, along with the ribbon 5, is conveyed through the transfer position R as the platen 2 rotates in the direction indicated by arrow D. During the transfer, the sheet 4 and the ribbon 5 are normally brought into close contact with each other by the guide roller 8, so they are separated by the separating roller 9 after passing through the transfer position R.
In forming a color image, however, the platen 2 needs to be rotated in the reverse direction (opposite to the direction of arrow D) to rewind the sheet 4 in order to transfer a second color top of a first color after the first color is transferred to the sheet 4. It is therefore necessary that the trailing end portion of the sheet 4 be held between the platen 2 and the pressure roller 6, as shown in FIG. 2. In this state, moreover, those portions of the sheet 4 and the ribbon 5 which have undergone the transfer operation are expected to have already been separated from each other by the separating roller 9. In consequence, as shown in FIG. 2, a nontransfer region or blank (corresponding to angle l.sub.0) is formed between that portion G of the sheet 4 located between the platen 2 and the separating roller 9 and that portion H of the sheet 4 between the platen 2 and the pressure roller 6.
Thus, the prior art image forming apparatus is subject to a drawback such that the nontransfer region (imageless region) of the sheet with the image thereon is wide, that is, the image forming region is narrow.